Human histocompatibility (HL-A) antigens have been extensively characterized by biologic and chemical means to gain a better understanding of the mechanism of allograft rejection and the expression of these antigens on the lymphoid cell surface in relation to other antigenic marker systems. The soluble HL-A antigens were utilized to produce potent, monospecific heteroantibodies in rabbits. Such reagents proved useful as immunoabsorbents for further antigen purification. Standardization of reagents for the lymphocytotoxic test led to more sensitive methods to detect mismatched donor- recipient pairs for kidney transplants. Extensive studies on the quantitative expression of HL-A antigens during the cell cycle showed no change during the in vitro lifespan of human fibroblasts. Some lymphoid cell lines derived from donors with lymphoid malignancies showed a preferential expression of HL-A antigens during the early log phase of cell growth. Methods were developed to effectively isolate soluble HL-A antigens from human platelets and sera. An effective method was developed for the iodination of proteins and cell surface utilizing solid state lactoperoxidase as a catalyst. A simple micromethod was also developed to measure blatogenesis of lymphoid cells providing a tool to study immune deficiency diseases.